Eutectic drying composition and method

ABSTRACT

Solid gas-drying forms are made by heating drying salts with a polar solvent to form a saturated solution, then cooling, freezing and forming. Preferred salts are combinations of calcium and lithium bromides and chlorides in particularly effective molar ratios.

This is a continuation of application Ser. No. 08/515,147, filed on Aug.15, 1995, now abandoned

TECHNICAL FIELD

This invention relates to compositions and methods for drying methaneand natural gas. Particular solid compositions and methods of makingthem in solid form are used in natural gas processing systems and forcompressed gas to be dried for use as automotive fuel.

BACKGROUND OF THE INVENTION

Natural gas contains small amounts of moisture which should be removedfor transmission through pipelines. Moisture specifications for gaswhich is to be used in fueling automotive fleets are generally moredifficult to meet than those for gas transmission lines. Drying ofnatural gas and methane continues to be a problem for these purposes andothers, and it appears that stringent demands will continue to be placedon suppliers of drying materials and devices.

Both solid and liquid dehumidifying, hygroscopic and dessicant materialshave long been known and studied. One of the more succinct disclosuresof combinations of drying salts appears in U.S. Pat. No. 2,143,008 toHeath and Minger. The inventors calculated and drew vapor pressureisobars on a chart representing ratios of concentrations in solution ofcalcium chloride, calcium bromide, lithium bromide, and lithiumchloride, thus showing the drying efficacy of certain combinations ofthese materials. They used the combinations in aqueous solutions, andstudied the effects of diluting them on their vapor pressures at varioustemperatures. Because the variable vapor pressures of these combinationsof salts are analogous to the melting temperatures of classical eutecticcompositions, I refer to them herein as "eutectic" drying compositions.

Liquid drying agents comprising calcium chloride are also disclosed byManning et al in U.S. Pat. No. 3,885,926; the solutions are used fordrying gaseous petroleum hydrocarbons.

Schilberg, in U.S. Pat. No. 2,988,509 combined sodium chloride andcalcium chloride in certain ratios to a "plastic" consistency, formedparticles, and dried them to concentrations of water less than 6percent. Pellets are made by Norton in U.S. Pat. No. 3,390,511, usingcompositions which may also include certain sugars. Small briquettes andpellets have been used widely in the art, but frequently they sufferfrom physical weaknesses which contribute to failures of the dryingsystems.

In U.S. Pat. No. 3,779,936, Pearce et al relate that the addition ofpolyethylene glycol in amounts from 0.1% to 10% in compositionscomprising calcium chloride will provide briquettes having greaterstrength and resistance to attrition than those without.

None of the aforementioned patents disclose eutectic or othercompositions together with physical forms thereof which are designed totake advantage of the actual process of dehydration of natural gas in apressurized line. Although the basics of the operation of a gas dryercontaining a charge of drying composition are known, the forms andcompositions have not in the past made the most of the physicalarrangement of the solid and liquid dessicants in the typical gas dryer,nor otherwise utilized the full advantage of the intimately mixed solidforms of my invention.

SUMMARY OF THE INVENTION

My invention is designed to utilize to the fullest advantage thephysical arrangement of a dry bed positioned above (downstream of) aliquid drying section in a gas dryer having a gas inlet below (upstream)of the liquid drying section, together with a solid form of salts whichare intimately mixed and formed by my unique process. Further, myinvention overcomes the disadvantages of prior art solid drying tabletsor pellets by achieving stronger and more intimately mixed solid forms.Unlike prior art solid forms, which were not intimately mixed andtherefore were physically weak because of different crystal sizes, mysolid forms are strong and, because of the intimate mixing, achieve analmost maximum eutectic effect.

My invention comprises in one aspect a "eutectic" composition in aparticular physical/chemical form, and the method of making thephysical/chemical form, and in another aspect the use of the eutecticform in a manner to efficiently remove moisture from a flowing gas.

My solid eutectic form is made as follows: (1) a combination of salts ischosen; the combination may be derived from the various synergisticcombinations of salts discussed in the above cited Heath and Minger U.S.Pat. No. 2,143,008, which is fully incorporated herein by reference, orit may comprise other salts having dessicant abilities such as thosementioned in the other patents cited above, and wherein the combinationhas a synergistic vapor pressure effect similar to that discussed in theHeath and Minger patent. (2) A small amount of water and/or a glycol orsimilar polar solvent is added and (3) the mixture is heated to make asolution. After the solution is formed (it may be supersaturated in theform of a slurry, but I prefer a simple saturated solution), (4) thesolution is poured into small molds such as ice cube trays or similarmolds having smaller cavities and placed in a freezer. (5) After theyare cooled and solidified, they are removed from the molds and placed inair-tight containers. It should be noted that no attempt is made toevaporate the water and/or the ethylene glycol although it is notessential that all of the solvent remain in the forms. Also it should beclear that the solution is poured into the molds when it is hot and thatit may begin to solidify around 200° F., for example, because of itssupersaturation.

Thus my gas dryer is a molded eutectic mixture of drying salts in asmall amount of a polar solvent.

My invention is based on the observation that moisture from a gascontacting the individual pellets or molded forms of the eutectic willtend to form a solution on the surface of the pellet or form, fullycoating the pellet or form before achieving enough volume to drip into agas/liquid contactor below. The solution which forms below has in thepast been generally more efficient at removing moisture than the solidmaterial above. Therefore the aim of my invention is to effectivelydistribute such a solution throughout the entire bed of pellets ormolded forms. Hence, my bed of pellets or molded forms is in effect asolid solution of the material rather than a dry form of it, and,because of the intimate mixture achieved by my method of preparation,maximizes the synergistic or eutectic effect of its components. Byallowing for the creation of a film of liquid solution of the salts onthe surface of the molded forms, my invention extends the capacity ofthe most efficient part of the unit. My invention will be described inmore detail below.

My invention includes a method of drying natural gas comprising passingsuch natural gas through a bed of solid forms of a composition derivedfrom lithium and calcium chlorides and bromides having the molarequivalents

    (Ca).sub.x (Li.sub.2).sub.(1-x) (Br.sub.2).sub.y (Cl.sub.2).sub.(1-y)

where x and y are independently selected from numbers from 0 to 1,provided that if either x or y is less than 0.1 or more than 0.9, theother must be a number from 0.1 to 0.9. Preferably the gas is alsopassed through a solution of said salts. Particular preferredcombinations of calcium, lithium, chloride, and bromide are describedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a gas dryer employing my molded drying forms.

FIG. 2 is a graph showing the performance of my molded eutecticcomposition in a gas dryer similar to that of FIG. 1.

FIG. 3 is a graph showing the performance of a second preferred moldedeutectic composition, also in such a gas dryer.

DETAILED DESCRIPTION OF THE INVENTION

My invention may employ any combination of drying salts which forms avapor pressure eutectic, such as the calcium and lithium chlorides andbromides discussed in the aforementioned Heath and Minger patent. Inparticular, I employ mixtures of two or more of the salts CaCl₂, CaBr₂,LiCl, and LiBr, said mixtures having as overall ratios of molarequivalents

    (Ca).sub.x (Li.sub.2).sub.(1-x) (Br.sub.2).sub.y (Cl.sub.2).sub.(1-y)(I)

where x and y are independently selected from numbers from 0 to 1,provided that if either x or y is less than 0.1 or more than 0.9, theother must be a number from 0.1 to 0.9.

There are two formulations I have adopted as preferred compositions, onecontaining bromine and chlorine, and one containing only chlorine.

The preferred composition without bromine is the mixture of lithium andcalcium chlorides at the point designated "G" in the figure of U.S. Pat.No. 2,143,008--that is, a mixture of lithium and calcium chlorideshaving Li₂ and Ca present in molar equivalents of 0.6 and 0.4. Minoramounts of polar solvent and defoamer are included, and the mixture isprepared and solidified as in the general procedure outlined below.

Thus this preferred composition may be expressed, based on the abovegeneral (I) formula where x=0.4 and y=0:

    (Ca).sub.0.4 (Li.sub.2).sub.0.6 (Cl.sub.2)

which may be generalized to a preferred range of compositions:

    (Ca).sub.x (Li.sub.2).sub.(1-x) (Br.sub.2).sub.y (Cl.sub.2).sub.1-y)

where x=0.2 to 0.6 and y=0 to 0.1

As a general procedure, an intimate mixture of the drying salts is madeby assuring dissolution of the salts by heating the mixture to at leastabout 220° F., preferably to about 250° F. or otherwise to achieve asaturated solution; the mixture is then cooled to freeze the mixture,preferably at about 0° F. I have found that the strength of the tabletor molded form is improved if temperatures below the freezing point ofthe brine are used. The molded forms are removed from the molds andimmediately placed in air-tight, preferably moisture-impermeable,containers for storage until they are put in use. The content of polarsolvent or carrier should be from 5% to 25% by weight, preferably10-20%. The polar solvent may be either water or a glycol but preferablycomprises water and ethylene glycol in a ratio of 1:10 to 10:1. It mayinclude other polar solvents or carriers such as propylene glycol,neopentyl glycol, other glycols, glycol ethers, sugars, and/orpolyhydric alcohols. The glycols act as solvents to bring about intimatemixing without reducing the drying capacity as additional water would.

A second preferred mixture is represented by point "19" on the diagramof the Heath and Minger U.S. Pat. No. 2,143,008. This point represents amixture of calcium and lithium chloride and bromide so as to providelithium and calcium as 0.6 and 0.4 molar equivalents respectively, andchlorine and bromine in proportions of 0.2 and 0.8 molar equivalentsrespectively. This preferred composition may be expressed using theabove general formula (I) where x=0.6 and y=0.8, which may begeneralized to a preferred range:

    (CA).sub.x (Li.sub.2).sub.(1-x) (Br.sub.2).sub.y (Cl.sub.2).sub.(1-y)

where x=0.4 to 0.8 and y=0.5 to 1.0.

Persons skilled in the art will recognize that various combinations oflithium and calcium chlorides and bromides can be used to achieve thedesired ratios of metal to metal and halide to halide for the abovedescribed second preferred composition.

In FIG. 1, the gas dryer construction is similar to prior artconstructions; however, it contains my molded drying forms which areable to perform more efficiently in the gas dryer environment than priorart dryer compositions. The gas dryer comprises a generally cylindricalbody 1 adapted to connect to a source of moisture-containing gas notshown through inlet 2 and to deliver relatively dry gas from an outlet3. A major portion of the upper half of the body 1 contains a bed 4 ofmy eutectic drying forms on a perforated or wire screen support 5. Thesupport 5 is constructed to permit both the upward passage of gas andthe downward trickling of water or brine, represented by droplets 6. Ata convenient distance above the inlet 2, a bubble cap tray 7 is placedto retard the further travel downward of liquid formed by droplets 6, sothat a pool 12 of water or brine may function as the liquid dryingsection. A drain 9 is provided so the removed liquid 8 may be discardedor recycled, and a fill port 10 is located at the top of the body 1.

In operation, moisture-laden gas, typically containing of the order of25-30 pounds of water per million standard cubic feet, enters inlet 2and passes through bubble-cap tray 7 to encounter the pool 12 above it.Pool 12 is made from droplets 6 which are generally near saturationbecause the eutectic drying forms in the lower part of bed 4 slowlydissolve and accumulate water as they pick up moisture from the gas.Although pool 12 is initially made up primarily of water from droplets6, and is therefore initially near saturation as are the droplets 6, thepool 12 acts to absorb moisture from the gas, and therefore the pool 12is constantly being diluted by new moisture from the gas. Nevertheless,because of the eutectic mixture of salts in the bed 4 carried into thepool 12, the pool 12 is a very efficient dehumidifier for the gaspassing through it. Accordingly, the upward moving gas in the spacebetween pool 12 and tray 7 contains perhaps thirteen pounds of moistureper million standard cubic feet of gas. As indicated above, the waterand/or other polar solvent content of the forms in the bed 4 mimics theconcentrated eutectic solutions which are so efficient in removingmoisture, and accordingly the lower part of bed 4 acts in a manner verysimilar to a pool, the surfaces of the eutectic forms constantly pickingup moisture, forming a eutectic salt solution, and dripping into thepool 12. The uppermost region of bed 4 is the driest and quite efficientat removing moisture from the gas which has already been well dried. Asthe lower region of bed 4 dissolves and drips into pool 12, more solidforms can be added to the top of bed 4. Fluid level control port 11controls the level of liquid 8.

FIG. 2 plots the outlet moisture content of natural gas treatedaccording to the first preferred composition of my invention againstoperating pressure at various temperatures. Natural gas was treated in adryer similar to that of FIG. 1, using a charge of molded dryercomposition made according to the procedure outlined above andcontaining the following ingredients:

    ______________________________________                                        Ingredient        Percent by Weight                                           ______________________________________                                        Calcium Chloride  52                                                          Lithium Chloride  28                                                          Ethylene Glycol   8.75                                                        Defoamer ("Callaway 3377")                                                                      2.5                                                         Water             8.75                                                        ______________________________________                                    

The curves were extrapolated from a number of points measured with aMiko Sunsprite ionization moisture analyzer. In addition to expressingthis composition in terms of molar equivalents as above, variations ofit may be expressed in terms of weight percent as calcium chloride about45-60%, lithium chloride about 24-32%, and solvent about 15-20%, whereinthe solvent may comprise water and ethylene glycol in ratios of about2:1 to 1:2.

FIG. 3 plots the outlet moisture content of natural gas treated with mysecond preferred composition made according to the procedure outlinedabove, in a dryer of a design similar to FIG. 1. The solid molded formsabout 3/8 inch in diameter and about 1/2 inch high contained thefollowing ingredients:

    ______________________________________                                        Ingredient       Percent by Weight                                            ______________________________________                                        Ethylene Glycol  10.5                                                         54% aqueous LiBr solution                                                                      22                                                           Lithium Chloride 7.9                                                          Calcium Bromide  59.6                                                         ______________________________________                                    

Curves in FIG. 3 were also drawn from a number of points measured with aMiko Sunsprite ionization moisture analyzer. This second preferredcomposition may also be expressed in terms of weight percent as about50-65% calcium bromide, about 8-15% lithium bromide, about 6-10% lithiumchloride, and the balance water, ethylene glycol, or mixtures thereof,preferably in ratios of 2:1 to 1:2.

It will be seen from FIGS. 2 and 3 that my invention is quite effectiveat removing moisture from natural gas, particularly at higher pressures,in a dryer of the type which utilizes the solid as well as the liquideutectic.

EXAMPLE 1

The operators of a gas drying tower in Pennsylvania were faced with aspecification for gas of a maximum of seven pounds of moisture permillion standard cubic feet. A commercial two-stage drying system usinga liquid dessicant similar to that disclosed by Heath and Minger in U.S.Pat. No. 2,143,008 was unable to reach this specification in the tower.The dessicant was replaced with molded forms of the first preferredcomposition mentioned above and utilized in a dryer similar to FIG. 1; amoisture content of six pounds per million standard cubic feet wasachieved.

I claim:
 1. Method of making a solid gas drying form comprising mixingdrying salts together with a small amount of polar solvent to make amixture, heating said mixture to form a saturated solution of salts, andthereafter cooling, freezing and forming said mixture as a frozen mass.2. Method of claim 1 wherein said drying salts comprise two or more ofthe salts lithium chloride, calcium chloride, lithium bromide andcalcium bromide.
 3. Method of claim 2 wherein said drying salts have, asmolar equivalents, the relationship

    (Ca).sub.x (Li.sub.2).sub.(1-x) (Br.sub.2).sub.y (Cl.sub.2).sub.(1-y)

where x and y are independently selected from numbers from 0 to 1,provided that if either x or y is less than 0.1 or more than 0.9, theother must be a number from 0.1 to 0.9.
 4. Method of claim 1 whereinsaid polar solvent comprises water and ethylene glycol.
 5. Method ofclaim 3 wherein x is a number from 0.2 to 0.6 and y is a number from 0to 0.1.
 6. Method of claim 3 wherein x is a number from 0.4 to 0.8 and yis a number from 0.5 to 1.0.
 7. Method of claim 1 wherein said formingtakes place in a mold.
 8. Method of claim 1 followed by placing saidfrozen mass in an airtight container.
 9. A solid gas-drying compositioncomprising an intimate mixture of about 45% to about 60% by weightcalcium chloride, about 24% to about 32% by weight lithium chloride, andabout 15% to about 20% of a polar solvent comprising water and ethyleneglycol in a ratio from about 2:1 to about 1:2, said intimate mixturebeing made by freezing a saturated solution of said calcium chloride andsaid lithium chloride in said polar solvent.
 10. A solid gas-dryingcomposition comprising an intimate mixture of about 50-65% by weightcalcium bromide, about 8-15% lithium bromide, about 6-10% lithiumchloride, and about 15% to about 25% polar solvent comprising water andethylene glycol in a ratio from about 2:1 to about 1:2, said intimatemixture being made by freezing a saturated solution of said calciumbromide, said lithium bromide and said lithium chloride in said polarsolvent.
 11. A molded solid gas drying form made by placing a mixture ofdrying salts having a vapor pressure eutectic together with a polarsolvent at a temperature elevated sufficiently to form a saturatedeutectic solution of said drying salts, placing said solution in a mold,and freezing said solution.
 12. A gas drying form of claim 11 whereinsaid salts are selected from calcium and lithium bromides and chlorides.13. A gas drying form of claim 11 wherein said elevated temperature isabout 220° F.